1. Field of the Invention
The present invention relates generally to aircraft flight simulators and more specifically to a digital radar landmass simulator system which includes a mapping radar portion and a terrain following radar portion that utilizes data collected for appropriate sweeps of the mapping radar portion.
2. Description of the Prior Art
Since the training of aircraft pilots in regular aircraft may be expensive, inconvenient and dangerous, aircraft flight simulators are employed. Flight simulators duplicate, insofar as is practical, the appearance of an aircraft cockpit, the view from the windows, the sensation of flying the aircraft and of course the various aircraft radars.
High performance aircraft, such as those which are designated F111, usually employ at least two radars, a mapping radar and a terrain following or terrain avoidance radar. The mapping radar typically scans a portion of the terrain located within an azimuth sector in front of the aircraft. From the radar energy reflected by this terrain the radar develops a display, or map, of the terrain for presentation, typically in a depressed center PPI (Plan Position Indicator) format. Although such a display is of considerable value to a pilot, it does not provide sufficient information as to the height of objects in front of the aircraft. For this reason and in order to develop an autopilot controlling signal, the mapping radar is usually supplemented by a terrain following radar.
The terrain following or terrain avoidance radar employs a narrow beam width antenna which is aligned in azimuth with the instantaneous flight path of the aircraft and which scans, or nods, up and down slightly faster than once per second in order to illuminate with microwave energy a sector area in front of the aircraft. Note that the term "sweep" is used herein to refer to the action of a single radar pulse and the corresponding reaction of the radar receiver to the pulse and reflected energy therefrom in developing a single line of the radar display. The term "scan" refers to the action of the antenna and the aggregate of many sweep lines.
From the microwave energy which is reflected back to the antenna the terrain following radar develops, on a rectangular display device, a visual representation of the height of the terrain in front of the aircraft.
Typically the terrain following radar display is calibrated along a vertical axis from minus thirty-two degrees to plus eight degrees corresponding to the deviation of the antenna below and above, respectively, a stabilized horizontal reference. The horizontal axis of the display is calibrated in log time, or log distance, usually to ten miles.
The display is used in conjunction with a "command template", or zero command line. The template follows the locus of points in depression angle and range to the clearance plane, in other words, the display which would occur if the aircraft were flown parallel to flat terrain at the clearance altitude. The template rises from the bottom of the display asymtotically approaching the zero degree line. At the maximum radar range, the template rises rapidly to the top of the display.
By watching the radar display of the terrain in conjunction with the template, the pilot can fly the aircraft so as to maintain the desired clearance height. More specifically, when all points of the radar display of the terrain are located to the right of the template, the aircraft is too high. Conversely, when any portion of the display of the terrain penetrates the template, in other words is located to the left of the template, the pilot is warned that a correction is necessary to maintain the clearance height and possibly to avoid striking the terrain.
In a similar fashion, the terrain following radar utilizes the radar return and the template to develop a autopilot controlling climb/dive signal which is used by an aircraft autopilot when it is used to fly the aircraft. The autopilot controlling climb/dive signal, which is also called a command angle signal and which is designated by the symbol .gamma..sub.c, is generated from a number of instantaneous command angle signals .gamma.'.sub.c, each of which are developed from a respective radar sweep. Typically, each instantaneous command angle signal is developed using analog circuitry, according to the formula: EQU .gamma.'.sub.c =.lambda.[TF.gamma..sub.sc +.theta..sub.a +(1.02H.sub.o /R.sub.s -F.sub.s +.GAMMA..sub.R)]
where
.lambda. is a system gain constant, PA1 TF.gamma..sub.sc is the instantaneous antenna elevation angle with respect to the aircraft, PA1 .theta..sub.a is the aircraft pitch angle, or attitude, with respect to the stabilized horizontal reference, (and which may differ from the actual flight path), PA1 H.sub.o is the clearance height selected by the pilot, PA1 R.sub.s is the slant range from the aircraft to the respective illuminated element of the terrain, PA1 F.sub.s is a function of the aircraft type, its flight path angle and velocity, and derived from the ride selection, i.e., softness/hardness, and PA1 .GAMMA..sub.R is the climb high function, typically the aircraft angle of attack (for positive angles only).
It should be noted that depending upon the implementation, the .theta..sub.a term may not be used, .theta..sub.a = 0, and the functional relationship of .GAMMA..sub.R may vary. The portion of the instantaneous command angle formula: EQU 1.02 H.sub.o /R.sub.s 32 F.sub.s
defines the command template, or zero command line.
The autopilot controlling command angle signal .gamma..sub.c is developed by taking the maximum of the instantaneous command angle signals Y'.sub.c which are developed over one half antenna scan cycle.
One prior art mapping radar simulator system includes an update computer which receives aircraft parameters that identify the position of the simulated aircraft and a scan computer which receives antenna parameters that describe the position of the simulated mapping radar antenna. From a high volume regional memory containing, in compressed format, parameters defining the terrain over the entire area which is to be simulated, a regional controller associated with the update computer transfers pertinent terrain parameters to a decompressor for expansion before they are stored in a district memory.
Responsive to the antenna parameters, a controller associated with the scan computer transfers terrain parameters for subsequent map radar sweeps from the district memory to a high speed sector memory. The controller retrieves from the sector memory those parameters which describe the terrain along a line which is coincident with the sweep line and which extends from the Nadir point, the point on the terrain directly below the aircraft, to the limit of the sector memory. From the retrieved parameters, a radar equation processor develops a signal for intensity modulating a mapping radar display to simulate a mapping radar presentation.
Since the mapping and the terrain following radars are usually independent aircraft systems, a logical, straightforward simulator implementation would utilize a terrain following radar simulator system which is independent of the mapping radar simulator system. As is no doubt apparent, the terrain following system would include many components similar to those found in the mapping radar system, a high volume memory to store the terrain parameters for the simulated area, a computer for controlling the flow of pertinent terrain parameters to a high speed memory and a radar equation processor for developing from these parameters a signal to intensity modulate a terrain following radar display.
To avoid the duplication of components present in the above described independent approach, a prior art multiplexed radar simulator system was developed. The multiplexed system takes advantage of the dead time which is present following each sweep of the mapping radar, a time in excess of that required for one sweep of the terrain following radar.
The multiplexed system includes a multiplexer for sequentially selecting antenna parameters from the mapping radar antenna and the terrain following radar antenna for subsequent processing by a system somewhat similar to those just described. The system alternately develops signals for intensity modulating a single sweep line of a mapping radar display and one for modulating a single sweep line of terrain following display. Finally, the intensity modulating signals are switched by a second multiplexer to the appropriate radar display.
It is important to note that each of the sweep lines which are used to drive the display only contain information relevant to the instantaneous position of the respective radar antenna, such as would be displayed on a single line of radar display. In other words, they do not convey information which is relevant to other radar antenna positions.
Further, it is interesting to note that the above described prior art system makes use of film as a storage medium for storing the terrain parameters in the regional memory and employs analog circuits for much of the signal processing.
Although greatly simplifying the design of radar simulators, this multiplexed system suffers from a major disadvantage in that although the dead time following each sweep line of the mapping radar will only support one sweep of the terrain following radar, such dead time periods occur at too slow a rate to support the normal number of terrain following sweeps. For this reason it is necessary to "trick" the viewer by synthesizing sweeps to supplement those which are lost. Most simply, this can be done by redisplaying the previous sweep a number of times, as necessary to fill in the display. Unfortunately when the mapping radar is in a long range mode, as few as one in ten of the terrain following sweeps may be processed.
Another major disadvantage of the multiplexed system is that the system must be designed as a whole rather than optimized for a particular radar system, thus, requiring a number of compromises. For example, if the sweep rate of the mapping radar is not an even multiple of the terrain following radar, it must be adjusted appropriately.
More information is available in:
1. "Project 1183, An Evaluation of Digital Radar Landmass Simulation", a paper by T. Hoog, R. Dahlberg and R. Robinson, 7th NTEC Ind. Conf., 11/74, Orlando, FL. 2. "RF-4E Aircraft DRLM Simulator", LP-5698, 3/78. 3. "F4F Weapons System Trainer Set, DRLMS Simulation Maint. Manuel", LP-5597, 9/73. 4. "F4-E(18) DLRMS Update System, Op. and Maint. Manuel", LP-5687, 3/77. Items 2-4 were printed by the Singer Company, Link Division, Sunnyvale, California.